The present invention generally relates to accurate beam pointing in the keyhole region of an airborne radio frequency (RF) antenna and, more particularly, to using phased array beam steering for third-axis motion in a two-axis gimbaled antenna control system.
Airborne radio frequency (RF) antenna terminal systems have been developed for the FAB-T (Family of Advanced Beyond line-of-sight Terminal) program for military EHF (Extremely High Frequency) satellite communication systems. Such RF antenna terminal systems may, for example, be mounted on a moving platform—such as a B-52 aircraft—and are designed to acquire and track a geostationary satellite payload or a polar satellite payload to establish a two-way digital beyond line-of-sight communication service that is secure, jam-resistant, scintillation-resistant (scintillation loss results from rapid variations in a communication signal's amplitude and phase due to changes in the refractive index of the Earth's atmosphere), and has a low probability of intercept and detection.
In order to meet the required communication link performance for such a communication service, the antenna pointing for tracking the satellite payload is required to be precisely controlled in the presence of platform motion. For example, the total signal loss due to antenna pointing error is typically required to be less than 1 decibel (dB), at the 3 sigma (standard deviation) level specified over a field-of-regard (FOR) given by 0 to 360 degrees in azimuth and 5 to 90 degrees in elevation.
One prior art RF antenna designed for existing EHF communication terminals used a two-axis gimbaled control system, which could not maintain the required pointing accuracy in the vicinity of the keyhole region—the region where the antenna pointing elevation angle is close to 90 degrees. Thus, in the keyhole region, the communication link could be temporarily lost due to pointing error using the two-axis gimbaled control system. A three-axis gimbaled control system was proposed and designed during the early phase of the FAB-T program to eliminate this keyhole problem. Because of the available antenna dome volume, however, the three-axis gimbaled control system could not accommodate the required antenna aperture to meet the desired antenna gain performance.
As can be seen, there is a need for accurate antenna pointing in the keyhole region from a moving platform. Moreover, there is a need for accurately pointing an antenna in the keyhole region of a moving platform that does not require a larger antenna dome, or a smaller antenna aperture.